I can't login to a specific vehicle in the Driver App that looks to be activated in the list of vehicles in the Admin Panel, why is this?

Make sure that the “Enabled” box for that vehicle is checked. You can do this yourself in the Vehicles section in the Admin Panel.

You can also check if the vehicle is reserved for a specific driver, because only the driver for whom the vehicle is reserved can see and select to log in to that vehicle in the list in the Driver App.